Method and apparatus for detecting oscillations in cardiac rhythm with electrogram signals

ABSTRACT

A cardiac rhythm management device is configured to detect oscillations in cardiac rhythm by comparing electrogram signals during successive heart beats. Upon detection of electrical alternans, the device may adjust its operating behavior to compensate for the deleterious effects of the condition.

FIELD OF THE INVENTION

[0001] This patent application pertains to methods and apparatus forcardiac rhythm management. In particular, it relates to the detectionsof oscillations in heart rhythm and its use by a cardiac rhythmmanagement device.

BACKGROUND

[0002] Cardiac rhythm refers to the temporal pattern of electrical ormechanical activity in the heart. Cardiac rhythm management devices areimplantable devices that provide electrical stimulation to selectedchambers of the heart in order to treat disorders of cardiac rhythm. Animplantable pacemaker, for example, is a cardiac rhythm managementdevice that paces the heart with timed pacing pulses. The most commoncondition for which pacemakers are used is in the treatment ofbradycardia, where the ventricular rate is too slow. Atrio-ventricularconduction defects (i.e., AV block) that are permanent or intermittentand sick sinus syndrome represent the most common causes of bradycardiafor which permanent pacing may be indicated. If functioning properly,the pacemaker makes up for the heart's inability to pace itself at anappropriate rhythm in order to meet metabolic demand by enforcing aminimum heart rate. Cardiac rhythm management devices may also be usedto treat tachyarrhythmias where the heart rhythm is too fast. In a typeof pacing therapy called anti-tachycardia pacing, one or more pacingpulses are output during a cardiac cycle in an effort to interrupt thereentrant circuit causing a tachycardia. Other tachyarrhythmias such asfibrillation can be treated by devices that deliver acardioversion/defibrillation shock when the tachyarrhythmia is detected.

[0003] Also included within the concept of cardiac rhythm is the mannerand degree to which the heart chambers contract during a cardiac cycleto result in the efficient pumping of blood. For example, the heartpumps more effectively when the chambers contract in a coordinatedmanner. The heart has specialized conduction pathways in both the atriaand the ventricles that enable the rapid conduction of excitation (i.e.,depolarization) throughout the myocardium. These pathways conductexcitatory impulses from the sino-atrial node to the atrial myocardium,to the atrio-ventricular node, and thence to the ventricular myocardiumto result in a coordinated contraction of both atria and bothventricles. This both synchronizes the contractions of the muscle fibersof each chamber and synchronizes the contraction of each atrium orventricle with the contralateral atrium or ventricle. Without thesynchronization afforded by the normally functioning specializedconduction pathways, the heart's pumping efficiency is greatlydiminished. Patients who exhibit pathology of these conduction pathways,such as bundle branch blocks, can thus suffer compromised cardiacoutput.

[0004] Heart failure refers to a clinical syndrome in which anabnormality of cardiac function causes a below normal cardiac outputthat can fall below a level adequate to meet the metabolic demand ofperipheral tissues. It usually presents as congestive heart failure(CHF) due to the accompanying venous and pulmonary congestion. Heartfailure can be due to a variety of etiologies with ischemic heartdisease being the most common. Some heart failure patients suffer fromsome degree of AV block or are chronotropically deficient such thattheir cardiac output can be improved with conventional bradycardiapacing. Such pacing, however, may result in some degree ofuncoordination in atrial and/or ventricular contractions because pacingexcitation from a single pacing site spreads throughout the myocardiumvia the conducting muscle fibers of either the atria or the ventricles,and not the faster specialized conduction pathways as in a physiologicalheart beat. Most pacemaker patients can still maintain more thanadequate cardiac output with artificial pacing, but the diminishment incardiac output may be significant in a heart failure patient whosecardiac output is already compromised. Intraventricular and/orinterventricular conduction defects are also commonly found in heartfailure patients and can contribute to cardiac dysfunction by causingunsynchronized contractions during intrinsic beats. In order to treatthese problems, cardiac rhythm management devices have been developedthat provide electrical pacing stimulation to one or more heart chambersin an attempt to improve the coordination of atrial and/or ventricularcontractions, termed cardiac resynchronization therapy.

SUMMARY OF THE INVENTION

[0005] The present invention relates to an implantable cardiac rhythmmanagement device that is configured to detect oscillatory behavior inthe electrical rhythm of the heart, referred to as electrical alternans.Since electrical alternans is known to be potentially arrhythmogenic,the device may be further configured to adjust its operation when thecondition is detected. Such operation adjustments may relate to themanner in which either bradycardia pacing or anti-tachycardia pacing isdelivered. Electrical alternans is also highly correlated with systolicdysfunction of the heart. A cardiac rhythm management device maytherefore also be configured to compensate for the decrease in cardiacoutput when electrical alternans is detected by initiating or modifyingthe delivery of cardiac resychronization therapy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a system diagram of an exemplary cardiac rhythmmanagement device.

[0007]FIGS. 2A and 2B are exemplary electrogram waveforms that exhibitelectrical alternans.

[0008]FIG. 3 illustrates an exemplary implementation of an algorithm fordetecting electrical alternans.

DETAILED DESCRIPTION

[0009] Under certain circumstances, the pattern of electrical excitationof the human heart exhibits oscillations during successive heart beats.Such beat-to-beat oscillations may relate to the amplitude, duration,and/or morphology of myocardial action potentials as well as ofexternally recorded waveforms (e.g., an EKG) that reflect thedepolarization and repolarization of the myocardium during a heart beat.This phenomena is referred to as electrical alternans and is usuallyindicative of a pathological state in which potentially dangerouscardiac arrhythmias are more likely to occur. In one of its aspects, thepresent invention provides an implantable cardiac rhythm managementdevice that is configured to utilize its sensing channels to detectelectrical alternans and then to adjust its operation accordingly. Thesensing channels produce electrogram signals that reflect the electricalactivity at a local cardiac site near which a sensing electrode isdisposed, and such local electrical activity exhibits oscillationsduring electrical alternans. In one embodiment, the device detects thestart of depolarization and the end of repolarization (i.e.,corresponding to the Q wave and T wave, respectively, in a surface EKG)in an electrogram signal from a ventricular sensing channel and measuresthe time interval therebetween. The interval so measured corresponds tothe action potential duration of the local cardiac site. The device maythen detect the presence of electrical alternans when the differencebetween intervals measured during successive heart beats exceeds aspecified threshold value and the difference persists for a specifiedlength of time or number of beats. Other embodiments may process theelectrogram signal to determine if oscillations exist with respect toother features such as amplitude or waveform morphology and then detectelectrical alternans if the oscillations are of sufficient magnitude andduration. Electrical alternans may also be detected by processing ofelectrogram signals from atrial sensing channels.

[0010] As aforesaid, the presence of electrical alternans in anindividual may indicate that an arrhythmogenic condition exists, and itmay be deleterious for an implantable cardiac rhythm management deviceto deliver its therapy to the patient in a normal manner under suchcircumstances. In accordance with the invention, an implantable cardiacrhythm management device that detects electrical alternans in the mannerdescribed above is also configured to adjust its operating behavior whenalternans is detected. Such a device may deliver any kind of cardiacrhythm management therapy to the patient during normal conditionsincluding bradycardia pacing, anti-tachycardia pacing, and/orcardioversion/defibrillation. Since it is known that electricalalternans occurs above a critical threshold heart rate, for example,bradycardia pacing at a rate above that critical threshold whenelectrical alternans is present may aggravate the situation. In oneembodiment, a device configured to deliver bradycardia pacing adjustsits pacing rate to a lower value when electrical alternans is detected.Rapid heart rates may also more readily trigger arrhythmias when anarrhythmogenic condition such as electrical alternans is present. Inanother embodiment, a device configured to deliver anti-tachycardiapacing lowers the threshold heart rate at which such therapy isinitiated when electrical alternans is detected in order to reduce theprobability of an arrhythmia occurring.

[0011] Electrical alternans is also highly correlated to oscillations inthe mechanical function of the heart that result in alternations inpulse pressure, referred to as pulsus alternans. Pulsus alternans isgenerally taken by clinicians to indicate systolic dysfunction,particularly in the left ventricle. Since the pumping action of theheart is due to the electromechanical coupling between electricaldepolarization of myocardial cells and their mechanical contraction,electrical alternans and pulsus alternans may be differentmanifestations of the same underlying phenomena in certain cases. In anyevent, detection of electrical alternans means that it is highlyprobable that pulsus alternans is also present. As noted above, certaincardiac rhythm management devices are designed to deliver pacing therapyin a manner that improves the coordination of both ventricles (or bothatria) during systolic contractions, termed cardiac resynchronizationtherapy. The presence of pulsus alternans in such patients indicatesthat systolic function has been further impaired, and it may bebeneficial for a device configurable for delivering resynchronizationpacing to adjust its operating parameters to compensate for this whenelectrical alternans, serving as a surrogate for pulsus alternans, isdetected. For example, a device may be configured to deliver bradycardiapacing to one ventricle in a conventional manner or even no pacing undernormal conditions. If electrical alternans is detected, however, thedevice may be programmed to initiate resynchronization therapy by pacingboth ventricles or one ventricle at multiple sites. In anotherembodiment, a device configured to deliver resynchronization pacingduring normal conditions may adjust one or more operating parameterswhen electrical alternans is detected so that the resynchronizationpacing is modified. Examples of operating parameters that may be soadjusted are the biventricular delay interval between paces delivered tothe right and left ventricles and the atrio-ventricular delay intervalbetween an atrial pace or intrinsic sense and a subsequent ventricularpace.

[0012] 1. Exemplary Hardware Platform

[0013] Cardiac rhythm management devices are usually implantedsubcutaneously or submuscularly on a patient's chest and have leadsthreaded intravenously into the heart to connect the device toelectrodes used for sensing and pacing. Leads may also be positioned onthe epicardium by various means. A block diagram of a multi-sitepacemaker having three sensing/pacing channels is shown in FIG. 1. (Asthe term is used herein, a “pacemaker” should be taken to mean anycardiac rhythm management device, such as an implantablecardioverter/defibrillator, with a pacing functionality.) Pacemakerssense intrinsic cardiac electrical activity by means of internalelectrodes disposed near the chamber to be sensed. A depolarization waveassociated with an intrinsic contraction of the atria or ventricles thatis detected by the pacemaker is referred to as an atrial sense orventricular sense, respectively. In order to cause such a contraction inthe absence of an intrinsic beat, a pacing pulse (either an atrial paceor a ventricular pace) with energy above a certain pacing threshold isdelivered to the chamber. The controller of the pacemaker is made up ofa microprocessor 10 communicating with a memory 12 via a bidirectionaldata bus, where the memory 12 comprises a ROM (read-only memory) forprogram storage and a RAM (random-access memory) for data storage. Thecontroller can be implemented by other types of logic circuitry (e.g.,discrete components or programmable logic arrays) using a state machinetype of design, but a microprocessor-based system is preferable. Thecontroller is capable of operating the pacemaker in a number ofprogrammed modes where a programmed mode defines how pacing pulses areoutput in response to sensed events and expiration of time intervals. Atelemetry interface 80 is also provided for communicating with anexternal programmer.

[0014] The multiple sensing/pacing channels may be configured to deliverbradycardia pacing, cardiac resynchronization therapy, oranti-tachycardia pacing. Illustrated in FIG. 1 is a configuration withone atrial and two ventricular sensing/pacing channels for deliveringbiventricular pacing. The atrial sensing/pacing channel in FIG. 1comprises ring electrode 43 a, tip electrode 43 b, sense amplifier 41,pulse generator 42, and an atrial channel interface 40 whichcommunicates bidirectionally with the controller 10. The device also hastwo ventricular sensing/pacing channels that similarly include ringelectrodes 23 a and 33 b, tip electrodes 23 b and 33 b, sense amplifiers21 and 31, pulse generators 22 and 32, and ventricular channelinterfaces 20 and 30. Each channel thus includes a pacing channel madeup of the pulse generator connected to the electrode and a sensingchannel made up of the sense amplifier connected to the electrode. Thechannel interfaces include analog-to-digital converters for digitizingsensing signal inputs from the sensing amplifiers, registers that can bewritten to for adjusting the gain and threshold values of the sensingamplifiers, and, in the case of the ventricular and atrial channelinterfaces, registers for controlling the output of pacing pulses and/orchanging the pacing pulse amplitude. For each channel, the sameelectrode pair is used for both sensing and pacing. In this embodiment,bipolar leads that include two electrodes are used for outputting apacing pulse and/or sensing intrinsic activity. Other embodiments mayemploy a single electrode for sensing and pacing in each channel, knownas a unipolar lead. A MOS switching network 70 controlled by themicroprocessor is used to switch the electrodes from the input of asense amplifier to the output of a pulse generator.

[0015] The controller 10 controls the overall operation of the device inaccordance with programmed instructions stored in memory, includingcontrolling the delivery of paces via the pacing channels, interpretingsense signals received from the sensing channels, and implementingtimers for defining escape intervals and sensory refractory periods. Thesensing circuitry of the pacemaker detects a chamber sense, either anatrial sense or ventricular sense, when an electrogram signal (i.e., avoltage sensed by an electrode representing cardiac electrical activity)generated by a particular channel exceeds a specified detectionthreshold. Pacing algorithms used in particular pacing modes employ suchsenses to trigger or inhibit pacing. The time intervals between suchsenses can also be measured in order to detect tachyarrhythmias so thatappropriate therapy can be delivered by the device. As described below,the controller may also be programmed to detect electrical alternans andto adjust the manner in which pacing therapy is delivered by the deviceupon such detection.

[0016] 2. Detection of Electrical Alternans

[0017]FIGS. 2A and 2B show examples of electrogram waveforms thatexhibit electrical alternans. In FIG. 2A, the start of ventriculardepolarization and the start of ventricular repolarization are marked byan R wave and T wave, respectively. The interval between the R wave andT wave during a single cardiac cycle then corresponds to the actionpotential duration in the myocardial fibers at the electrode site. Onemanifestation of electrical alternans is a beat-to-beat oscillation inthe action potential duration as reflected by the successive differencesin the R-T interval. FIG. 2B shows another example of an electrogramwaveform in which the amplitude of the T wave oscillates from beat tobeat.

[0018] In order to detect electrical alternans, the controller isprogrammed to extract some feature from the electrogram waveform duringsuccessive cardiac cycles and determine if a beat-to-beat oscillationexists with respect to that feature. Examples of such features includethe R-T interval and T wave amplitude as noted above. In order to detectelectrical alternans with specificity, specified threshold values may beemployed so that the magnitude of the beat-to-beat oscillation must beof a certain magnitude and must persist for a certain number of heartbeats or period of time before electrical alternans is detected.

[0019]FIG. 3 shows an exemplary algorithm for detecting electricalalternans based upon measurement of the R-T interval that can beimplemented by the controller. At step S1, the device waits for an Rwave (i.e., a ventricular sense). When an R wave is detected, the devicethen looks for a T wave in the ventricular sensing channel during anappropriate time window at step S2. When both R and T waves are detectedduring a cardiac cycle, the time interval between the two is measured atstep S3. The difference between the R-T interval of the present cardiaccycle and the R-T interval of the previous cardiac cycle is thencomputed at step S4. If the R-T interval difference exceeds a specifiedthreshold, as determined at step S5, a counter that keeps track of thenumber of consecutive R-T interval differences that exceed the thresholdis incremented at step S8. Otherwise, a condition of no electricalalternans is detected at step S6, the counter is reset at step S7, andthe device waits for the next R wave at step S1. If the R-T intervaldifference does exceed the threshold, after incrementing the counter atstep S8, the counter's value is compared to a threshold count value atstep S9. The threshold count value specifies the number of consecutiveheart beats that the R-T interval difference must be above thresholdbefore electrical alternans is detected. If the count exceeds the countthreshold value, electrical alternans is detected at step S10. Thedevice then returns to step S1 and measures the next R-T interval.

[0020] 3. Adjustment of Bradycardia Pacing Rate

[0021] Bradycardia pacing modes refer to pacing algorithms used to pacethe atria and/or ventricles when the intrinsic atrial and/or ventricularrate is inadequate due to, for example, AV conduction blocks or sinusnode dysfunction. Such modes may either be single-chamber pacing, whereeither an atrium or a ventricle is paced, or dual-chamber pacing inwhich both an atrium and a ventricle are paced. Modern pacemakers aretypically programmable so that they can operate in any mode which thephysical configuration of the device will allow. Additional sensing ofphysiological data allows some pacemakers to change the rate at whichthey pace the heart in accordance with some parameter correlated tometabolic demand, called rate-adaptive pacing. Measurement of minuteventilation or body activity can be used to estimate metabolic demandfor this purpose.

[0022] Pacemakers can enforce a minimum heart rate either asynchronouslyor synchronously. In asynchronous pacing, the heart is paced at a fixedrate irrespective of intrinsic cardiac activity. There is thus a riskwith asynchronous pacing that a pacing pulse will be deliveredcoincident with an intrinsic beat and during the heart's vulnerableperiod which may cause fibrillation. Most pacemakers for treatingbradycardia today are therefore programmed to operate synchronously in aso-called demand mode where sensed cardiac events occurring within adefined interval either trigger or inhibit a pacing pulse. Inhibiteddemand pacing modes utilize escape intervals to control pacing inaccordance with sensed intrinsic activity. In an inhibited demand mode,a pacing pulse is delivered to a heart chamber during a cardiac cycleonly after expiration of a defined escape interval during which nointrinsic beat by the chamber is detected. If an intrinsic beat occursduring this interval, the heart is thus allowed to “escape” from pacingby the pacemaker. Such an escape interval can be defined for each pacedchamber. For example, a ventricular escape interval can be definedbetween ventricular events so as to be restarted with each ventricularsense or pace. The inverse of this escape interval is the minimum rateat which the pacemaker will allow the ventricles to beat, sometimesreferred to as the lower rate limit (LRL).

[0023] In atrial tracking and/or atrio-ventricular sequential pacing,another ventricular escape interval is defined between atrial andventricular events, referred to as the atrio-ventricular interval (AVI).The atrio-ventricular interval is triggered by an atrial sense or paceand stopped by a ventricular sense or pace. A ventricular pace isdelivered upon expiration of the atrio-ventricular interval if noventricular sense occurs before. Atrial-tracking ventricular pacing andatrio-ventricular sequential pacing attempt to maintain theatrio-ventricular synchrony occurring with physiological beats wherebyatrial contractions augment diastolic filling of the ventricles. If apatient has a physiologically normal atrial rhythm, atrial-trackingpacing also allows the ventricular pacing rate to be responsive to themetabolic needs of the body. A pacemaker can also be configured to pacethe atria on an inhibited demand basis, where an atrial escape intervalis then defined as the maximum time interval in which an atrial sensemust be detected after a ventricular sense or pace before an atrial pacewill be delivered. The LRL in that case is the sum of the atrial escapeinterval and the AVI.

[0024] Electrical alternans is known to occur only above a certaincritical threshold heart rate that varies with the individual patient. Apacemaker operating in an inhibited demand mode, by enforcing aspecified minimum heart rate, can be responsible in some cases formaintaining a heart rate that allows electrical alternans to occur. Thecontroller may therefore be programmed to decrease the LRL by aspecified amount upon detection of electrical alternans. Such a decreasein the LRL may also be beneficial even if the electrical alternanspersists by making the triggering of an arrhythmia less likely.

[0025] 4. Adjustment of Anti-Tachycardia Pacing

[0026] The cardiac rhythm management device of FIG. 1 may be programmedwith a plurality of selectable ATP pacing protocols that define themanner in which anti-tachycardia pacing is delivered. In amicroprocessor-based device, the output of pacing pulses is controlledby a pacing routine that implements the selected pacing protocol asdefined by various parameters. A data structure stored in memorycontains the parameter sets that define each of the available pacingprotocols. Pacing protocols for ATP therapy can generally be dividedinto two classes: those that deliver one or more pulses in timedrelation to detected depolarizations and those that deliver a continuouspulse train for a specified time beginning after a detecteddepolarization. Both types of ATP protocols attempt to block thereentrant depolarization wavefront causing the tachycardia with a seconddepolarizing wavefront produced by a pacing pulse. Protocols of thefirst group may vary according to parameters that define the number ofpulses delivered and the particular timing employed. Protocols of thesecond group include so-called burst pacing in which a short train ofpulses is delivered for a specified time and may vary according toparameters that define the duration, frequency, and timing of thepulses.

[0027] The device delivers ATP therapy or a defibrillation shock underprogrammed control of the microprocessor in response to sensed activityfrom the sensing channels. A sensing routine analyzes the electricalactivity received from the sensing channels in order to detect atachyarrhythmia, and the tachyarrhythmia is then classified as atachycardia (i.e., a terminable tachyarrhythmia) or fibrillation basedupon rate and/or other criteria. The device detects a ventriculartachyarrhythmia, for example, by counting ventricular senses receivedvia the ventricular sensing channel in order to measure the heart rateand determine whether the rate exceeds a selected threshold value. Oncea tachyarrhythmia is detected, the rhythm is classified into either atachycardia or a fibrillation zone by comparing the heart rate to afibrillation rate boundary or by other means such as assessing thestability of the rhythm. If the tachyarrhythmia is classified asterminable, a pacing routine executed by the microprocessor delivers ATPpulses in accordance with the parameters of a selected protocol.

[0028] As noted above, the object of anti-tachycardia pacing is tocreate a pace-induced wavefront that propagates into the re-entrantcircuit of the tachycardia and extinguishes it. Different protocols areapt to be more successful than others in terminating particulartachyarrhythmias that may differ as to rate and/or depolarizationpattern. For this reason, modem cardiac rhythm management devices arecapable of employing a number of different ATP protocols to delivertherapy where pacing parameters affecting the magnitude and timing ofthe pulses can also be adjusted for each protocol. Ideally, a clinicianwould program the device to deliver pacing therapy using a protocol andparameters that will perform best for a particular patient'stachyarrhythmia.

[0029] Upon detection of electrical alternans, the controller may beprogrammed to adjust the manner in which anti-tachycardia pacing isdelivered that takes account of the greater potential for onset of anarrhythmic episode. In one example, the tachyarrhythmia rate thresholdat which anti-tachycardia pacing is initiated is decreased so that theanti-tachycardia therapy is delivered sooner than in a normal mode ofoperation. Clinical testing of an individual patient may also revealthat certain anti-tachycardia pacing protocols are more successful thanothers in terminating a tachycardia preceded by electrical alternans butless successful in terminating a tachycardia not preceded by electricalalternans. In those cases, the controller can be programmed to adjustthe particular anti-tachycardia pacing protocol to be used forterminating a tachycardia and/or particular parameters defining thatprotocol when electrical alternans is detected.

[0030] 5. Adjustment of Cardiac Resynchronization Therapy

[0031] Cardiac resynchronization therapy is pacing stimulation appliedto one or more heart chambers in a manner that restores or maintainssynchronized contractions of the atria and/or ventricles and therebyimproves pumping efficiency. Certain patients with conductionabnormalities may experience improved cardiac synchronization withconventional single-chamber or dual-chamber pacing as described above.For example, a patient with left bundle branch block may have a morecoordinated contraction of the ventricles with a pace than as a resultof an intrinsic contraction. Resynchronization pacing, however, may alsoinvolve delivering paces to multiple sites of a heart chamber or pacingboth ventricles and/or both atria in accordance with a resynchronizationpacing mode as described below. Ventricular resynchronization pacing isuseful in treating heart failure because, although not directlyinotropic, resynchronization results in a more coordinated contractionof the ventricles with improved pumping efficiency and increased cardiacoutput. Resynchronization pacing of the atria may also be beneficial incertain heart failure patients, particularly for preventing the onset ofatrial arrhythmias.

[0032] One way to deliver resynchronization therapy is to pace a sitewith a synchronous bradycardia pacing mode and then deliver one or moreresynchronization paces to one or more additional pacing sites in adefined time relation to one or more selected sensing and pacing eventsthat either reset escape intervals or trigger paces in the bradycardiapacing mode. One such resynchronization pacing mode may be termed offsetresynchronization pacing. In this mode, a first site is paced with abradycardia mode, and a second site receives a resynchronization pace atan offset interval with respect to the pace delivered to the first site.The offset interval may be zero in order to pace both sitessimultaneously, positive in order to pace the first site after thesecond, or negative to pace the first site before the second. Forexample, in biventricular resynchronization pacing, one ventricle ispaced with a bradycardia mode while the contralateral ventricle receivesresynchronization paces at the specified biventricular offset interval.The offset interval would normally be individually specified to optimizecardiac output in a particular patient. Ventricular resynchronizationcan also be achieved in certain patients by pacing at a singleunconventional site, such as the left ventricle instead of the rightventricle. In such a mode, right ventricular senses may be used totrigger left ventricular paces or used to define an escape interval thatupon expiration causes delivery of a left ventricular pace.

[0033] Cardiac rhythm management devices for deliveringresynchronization therapy may be configured in a number of differentways and with a number of different parameter settings. These parameterscan be initially programmed after implantation while a physician ismonitoring the patient so that the resynchronization therapy isdelivered optimally. When the pumping efficiency of the patient's heartdeteriorates as may be indicated by detection of an oscillatory rhythm,however, modification of those parameters may be necessary for continuedoptimal treatment. Accordingly, the controller may be programmed tomodify its resynchronization pacing parameters upon detection ofelectrical alternans, with the exact manner in which such parameters aremodified depending upon the individual patient's condition. Suchparameter modifications may result in, for example, initiation ofresynchronization pacing when such pacing is not normally delivered bythe device, reconfiguration of pacing sites so that different cardiacsites are paced, adjustment of a biventricular offset interval forbiventricular pacing modes, and adjustment of the atrio-ventricularinterval for resynchronization pacing modes that employ atrial trackingor atrio-ventricular sequential pacing.

[0034] Although the invention has been described in conjunction with theforegoing specific embodiments, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. An implantable cardiac rhythm management device,comprising: a sensing channel for generating an electrogram signalreflecting local electrical activity at a cardiac site near which anelectrode is disposed; and a controller for processing the electrogramsignal to detect oscillations in the signal during successive heartbeats and for detecting electrical alternans when such oscillationsexceed a specified magnitude; wherein the controller is programmed toadjust the operating behavior of the device when electrical alternans isdetected.
 2. The device of claim 1 wherein the controller is programmedto detect oscillations in a measured time interval between the start ofdepolarization and the end of repolarization as reflected in theelectrogram signal in order to detect electrical alternans.
 3. Thedevice of claim 1 wherein the controller is programmed to detectoscillations in an amplitude of a portion of the electrogram signal inorder to detect electrical alternans.
 4. The device of claim 1 furthercomprising: a pacing channel; and wherein the controller is programmedto deliver pacing therapy in accordance with a bradycardia mode, andfurther wherein the controller is programmed to decrease a programmedpacing rate when electrical alternans is detected.
 5. The device ofclaim 1 further comprising: a pacing channel; and wherein the controlleris programmed to deliver anti-tachycardia pacing therapy when a measuredheart rate exceeds a specified tachycardia threshold rate, and furtherwherein the controller is programmed to decrease the tachycardiathreshold rate when electrical alternans is detected.
 6. The device ofclaim 1 further comprising: a first ventricular pacing channel; a secondventricular pacing channel; and wherein the controller is programmed todeliver pacing therapy in accordance with a bradycardia mode, andfurther wherein the controller is programmed to initiate ventricularresynchronization pacing by delivering paces through both ventricularpacing channels when electrical alternans is detected.
 7. The device ofclaim 1 further comprising: a first ventricular pacing channel; a secondventricular pacing channel; and wherein the controller is programmed todeliver ventricular resynchronization pacing by delivering paces throughboth ventricular channels, and further programmed to adjust aresynchronization operating parameter when electrical alternans isdetected.
 8. The device of claim 7 wherein the resynchronizationoperating parameter adjusted by the controller is an atrio-ventricularinterval.
 9. The device of claim 7 wherein the resynchronizationoperating parameter adjusted by the controller is a biventricular delayinterval.
 10. The device of claim 1 further comprising: a first atrialpacing channel; a second atrial pacing channel; and wherein thecontroller is programmed to deliver pacing therapy in accordance with abradycardia mode, and further wherein the controller is programmed toinitiate atrial resynchronization pacing by delivering paces throughboth atrial channels when electrical alternans is detected.
 11. A methodfor operating an implantable cardiac rhythm management device,comprising: generating an electrogram signal reflecting local electricalactivity at a cardiac site near which an electrode is disposed;processing the electrogram signal to detect oscillations in the signalduring successive heart beats and detecting electrical alternans whensuch oscillations exceed a specified magnitude; and adjusting theoperating behavior of the device when electrical alternans is detected.12. The method of claim 11 further comprising detecting oscillations ina measured time interval between the start of depolarization and the endof repolarization as reflected in the electrogram signal in order todetect electrical alternans.
 13. The method of claim 11 furthercomprising detecting oscillations in an amplitude of a portion of theelectrogram signal in order to detect electrical alternans.
 14. Themethod of claim 11 further comprising delivering pacing therapy inaccordance with a bradycardia mode and decreasing a programmed pacingrate when electrical alternans is detected.
 15. The method of claim 11further comprising delivering anti-tachycardia pacing therapy when ameasured heart rate exceeds a specified tachycardia threshold rate anddecreasing the tachycardia threshold rate when electrical alternans isdetected.
 16. The method of claim 11 further comprising initiatingventricular resynchronization pacing by delivering paces to bothventricles when electrical alternans is detected.
 17. The method ofclaim 11 further comprising delivering ventricular resynchronizationpacing by delivering paces through both ventricular channels andadjusting a resynchronization operating parameter when electricalalternans is detected.
 18. The method of claim 17 wherein the adjustedresynchronization operating parameter is an atrio-ventricular interval.19. The method of claim 17 wherein the adjusted resynchronizationoperating parameter is a biventricular delay interval.
 20. The method ofclaim 11 further comprising initiating atrial resynchronization pacingby delivering paces through both atrial channels when electricalalternans is detected.